Noninvasive measuring method for rapid temperature variation under dc excitation magnetic field

ABSTRACT

Provided is a noninvasive measuring method for rapid temperature variation under a DC excitation magnetic field, comprising: (1) positioning ferromagnetic particles at a measured object; (2) applying a DC magnetic field to area of the ferromagnetic particles enabling the ferromagnetic particles to reach saturation magnetization state; (3) obtaining steady temperature T 1  of the measured object at room temperature, and calculating initial spontaneous magnetization M 1 , of the ferromagnetic particles according to the steady temperature T 1 ; (4) detecting amplitude A of a magnetization variation signal of the ferromagnetic particles after temperature of the measured object varies, and calculating temperature T 2  after change according to the amplitude A of the magnetization variation signal; and (5) calculating temperature variation ΔT=T 2 -T 1  according to the temperature T 2  after change and the steady temperature T 1 . The present invention can realize noninvasive temperature measurement with high speed and high accuracy so as to resolve technical problems of low speed and low precision.

FIELD OF THE INVENTION

The invention relates to a technical field of rapid and accuratetemperature measurement, particularly to a noninvasive measuring methodfor rapid temperature variation under a DC excitation magnetic field,and more more particularly to a noninvasive temperature measuring methodunder a DC excitation magnetic field based on relationship betweensaturation magnetization of ferromagnetic particles and temperaturefeaturing high time definition and high temperature definition.

BACKGROUND OF THE INVENTION

Temperature is one of the most basic physical quantities in the natureand temperature measurement is of great importance for cognition ofnatures of materials in the nature. Rapid temperature measuring methodusing ferromagnetic particles is a brand new temperature measuringmethod calculating temperature by detecting magnetization varies offerromagnetic particles and by certain model relationship, and featuringnon-invasion, ultrahigh speed (on nanosecond level) and high accuracy.The temperature measuring method using ferromagnetic particles may bewidely used in fields such as laser heating, rapid metal solidificationand temperature measurement of motors for its non-invasion and highspeed.

Development of engineering technology brings in many heat conductionproblems such as ultrashort duration of heat effect, ultrahigh densityof transient heat flux and ultrafast temperature variation. ConventionalFourier's Law is no longer applicable for those extraordinary heatconductions of ultrahigh speed, and heat conduction effect not followingthe Fourier's Law occurring under the extraordinary heat conductions isknown as the non-Fourier heat conduction effect. Unfortunately, it isdifficult for techniques and devices in prior art to accurately detecttemperature variation in such short durations. The problem of ultrashortaction duration may be overcome by rapid and noninvasive temperaturemeasurement by ferromagnetic particles and the temperature variationprocess is monitored for further research.Particular temperature measuring problems such as pulsating flametemperature measurement in an engine combustion chamber of an aircraftand temperature measurement of high-temperature thermal processingfurnace, and welding and casting by high-frequency heating often occurin the field of aerospace, which cannot be resolved effectively byconventional temperature measuring method. Accordingly, temperaturemeasuring devices should feature fast response and high accuracy whichcan be realized by combining noninvasive rapid temperature measurementby ferromagnetic particles and temperature conduction. Therefore,Noninvasive rapid measuring technology with high accuracy is still anurgent problem to be resolved.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, it is an objective of theinvention to provide a noninvasive temperature measuring method under aDC excitation magnetic field based on relationship between saturationmagnetization of ferromagnetic particles and temperature featuring hightime definition and high temperature definition, so as to realizenoninvasive temperature measurement with high speed and high accuracyand resolve technical problems of low speed and low precision.To achieve the above objective, there is provided a noninvasivemeasuring method for rapid temperature variation under a DC excitationmagnetic field, comprising steps of:

-   -   (1) positioning ferromagnetic particles at a measured object;    -   (2) applying a DC magnetic field to area of the ferromagnetic        particles enabling the ferromagnetic particles to reach        saturation magnetization state;    -   (3) obtaining steady temperature T₁ of the measured object at        room temperature, and calculating initial spontaneous        magnetization M₁ of the ferromagnetic particles according to the        steady temperature T₁;    -   (4) detecting amplitude A of a magnetization variation signal of        the ferromagnetic particles after temperature of the measured        object varies, and calculating temperature-after-variation T₂        according to the amplitude A of the magnetization variation        signal; and    -   (5) calculating temperature variation ΔT=T₂−T₁ according to the        temperature-after-variation T₂ and the steady temperature T₁.        In a class of the embodiment, in step (1), the ferromagnetic        particles are placed inside the measured object or coated on the        surface of the measured object. In a class of the embodiment, in        step (3), the steady temperature T₁ of the measured object at        room temperature is obtained by a thermocouple or an optical        fiber temperature sensor, and the initial spontaneous        magnetization M₁ of the ferromagnetic particles at temperature        T₁ is calculated according to a curve between saturation        magnetization and temperature of the ferromagnetic particles.        In a class of the embodiment, in step (4), the step of        calculating temperature T₂ after change according to amplitude A        of the magnetization variation signal further comprises:        in terms of a relationship between temperature T₂ after change        and amplitude A of the magnetization variation signal:

${A = {\frac{a\; \beta \; {NS}}{\Delta \; t}*\left\{ {{{M\left( {T = 0} \right)} \cdot \left\lbrack {1 - {s\left( \frac{T_{2}}{T_{c}} \right)}^{\frac{3}{2}} - {\left( {1 - s} \right)\left( \frac{T_{2}}{T_{c}} \right)^{\frac{5}{2}}}} \right\rbrack^{\frac{1}{3}}} - M_{1}} \right\}}},$

calculating temperature T₂ after change according to amplitude A of themagnetization variation signal, wherein α is the proportionalcoefficient of magnetization variation ΔB to spontaneous magnetizationvariation ΔM, β is amplification factor of a test circuit, N is turns ofan inductance coil, S is inner area of the inductance coil, Δt isduration of the temperature changing process, M(T=0) is spontaneousmagnetization of the ferromagnetic particles at absolute zerotemperature, s is a parameter of the thermal demagnetization curve of aferromagnetic material, T_(c) is Curie temperature of the ferromagneticparticles, and M(T=0) and T_(c) are determined for a definedferromagnetic particle material, and M₁ is the initial spontaneousmagnetization of the ferromagnetic particles at temperature T₁.In a class of the embodiment, in step (4), the step of detectingamplitude A of the magnetization variation signal of the ferromagneticparticles after temperature of the measured object varies furthercomprises:detecting the magnetization variation signal of the ferromagneticparticles in the measured area using two identical single-layer coils assensors, wherein one inductance coil ^(α) is used as a detecting coil,the measured object is contained therein so that the coil can detect allmagnetization variation signals of the measured object, the otherinductance coil γ is placed at a symmetrical position in the DC magneticfield using as a reference coil, which receives noise in thecircumstance instead of induction signals of the measured object, and amagnetization variation signal of the ferromagnetic particles isdetected by inductance coil ^(α), which is processed by a conditioningcircuit including a differential amplification circuit along with asignal detected by inductance coil γ, whereby detecting amplitude A ofthe processed magnetization variation signal.Advantages of the Invention comprise: 1. The invention realizesnoninvasive measurement: compared with invasive temperature measuringmethod, which causes comparatively large damages and tends to change orinterfere with properties of the measured object by probes in spite ofbeing simple and convenient to monitor the temperature in real timeaccurately and directly, noninvasive temperature measuring method isable to realize high accurate measurement under the condition of beingalmost physically isolated from the measured object. 2. The inventionrealizes fast measurement: it is impossible to realize temperaturemeasurement below microsecond in the prior art, comparatively, sincetheoretical hysteresis caused by spontaneous magnetization offerromagnetic particles changing with temperature is extremely small(about 10 picoseconds), the invention can realize temperaturemeasurement under temperature variation caused by heat conduction on theabove timescale. 3. The invention realizes high accurate measurement:for the test signal corresponds to magnetization variation in themeasurement, integral calculation can suppress noise in the measurementeffectively in the process of obtaining the temperature so as to enablehigher accuracy in temperature measurement.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a flow chart of a measuring method for rapid temperaturevariation according to the present invention;

FIG. 2 shows a curve between saturation magnetization and temperature ofthe ferromagnetic particles according to one embodiment of the presentinvention;

FIG. 3 shows an equivalent model of an inductance coil at highfrequencies according to one embodiment of the present invention;

FIG. 4 shows amplitude-frequency response of an equivalent model of aninductance coil at high frequencies;

FIG. 5 shows response to a single laser impulse detected by aphotovoltaic power diode according to one embodiment of the presentinvention;

FIG. 6 shows response to a single thermal impulse detected by a coilaccording to one embodiment of the present invention;

FIG. 7 shows response to a thermal change in 1 ms detected by a coilaccording to one embodiment of the present invention;

FIG. 8 shows temperature of thermal change in 1 ms detected by athermocouple according to one embodiment of the present invention;

FIG. 9 shows temperature in 1 ms calculated by the method of the presentinvention according to one embodiment of the present invention;

FIG. 10 show a comparison chart of temperature in 1 ms detected by themethod of the present invention and by a thermocouple; and

FIG. 11 shows difference of temperature in 1 ms detected by the methodof the present invention and by a thermocouple.

SPECIFIC EMBODIMENTS OF THE INVENTION

For clear understanding of the objectives, features and advantages ofthe invention, detailed description of the invention will be given belowin conjunction with accompanying drawings and specific embodiments. Itshould be noted that the embodiments are only meant to explain theinvention, and not to limit the scope of the invention.

Principle of temperature measurement of ferromagnetic particles isintroduced briefly at first for better understanding of the presentinvention.When size of a ferromagnetic particle reduces to a certain scale, itsferromagnetic property converts to paramagnetic property and itsmagnetic property can be described by the Langevin's function:M=φM_(s)(coth(mH/kT)−kT/mH), where M_(s) is saturation magnetic momentof magnetic nanoparticles, m is averaged magnetic moment of the magneticnanoparticles, φ is mass of the magnetic nanoparticles (number of themagnetic nanoparticles), k is the Boltzmann's constant, H is anexcitation magnetic field, and T is an absolute temperature. PerformingTaylor expansion on the Langevin's function, using the AC model, thetemperature can be calculated by detecting harmonic waves. However, itis hard to realize measurement with high time definition for it requiresthat frequency of the AC excitation magnetic field applied to theparamagnetic particles is high enough and magnetization thereof does notattenuate. Response of the paramagnetic nanoparticles is extremely weakand is hard to detect using the DC model. Relationship betweenspontaneous magnetization of ferromagnetic particles and temperature isdetermined and can be described by an equation m(τ), no delay occurs inspontaneous magnetization changing with temperature, and thereforeselecting ferromagnetic particles as temperature sensitive elementsmeets the requirement of high time definition in temperaturemeasurement.For ferromagnetic particles, macro magnetization thereof is formed bydistribution of spontaneous magnetization of magnetic domains thereinand can be described by the following equation:

${M = {\sum\limits_{i = 1}^{n}\; {M_{s}\mspace{14mu} \cos \mspace{14mu} \theta_{i}}}},$

where θ₁ is an angle between an ith spontaneous magnetization and themagnetic field, and M, is spontaneous magnetization.Performing temperature measurement by the relationship between residualmagnetization and temperature, relationships between residualmagnetization and spontaneous magnetization for crystals of differentcrystal systems are as follows:

polycrystal of uniaxial crystal system M_(r) = 0.5M_(s) polycrystal oftriaxial crystal system (K₁ > 0) M_(r) = 0.832M_(s) polycrystal ofquadriaxial crystal system (K₁ < 0) M_(r) = 0.866M_(s)The above residual magnetization is obtained by reducing the excitationmagnetic field to 0 slowly at a state of saturation magnetization. Inpractice, polycrystal operates on the demagnetization curve in thesecond quadrant, which makes the relationship between residualmagnetization and temperature more complicated. However, undersaturation magnetic field, spontaneous magnetizations point to themagnetic field, and macro magnetization at the moment is linearsuperposition of spontaneous magnetizations, namely M=M_(s). Therefore,to perform temperature measurement with high time definition, a magneticfield should be applied to the ferromagnetic particles enabling them toreach saturation magnetization state at first, and magnetic responsecaused by a temperature variation is detected, whereby obtaining thetemperature variation. Spontaneous magnetization M_(s) is the most basiccharacter of ferromagnetic materials. In the last century, people tookgreat efforts to theoretically describe the function of spontaneousmagnetization M_(s) with respect to temperature, which ranges from theabsolute zero temperature to the Curie temperature. At present, onlysaturation magnetization (0<τ<1) at T=0 can be estimated, namely M_(o)calculated based on Density Functional Theory matches best with M_(o)obtained by experiments in practice. The Curie temperature T_(c) iscalculated based on Density Functional Theory of classical Heisenbergmodel and Langevin's Rotating Dynamics Theory in some other researches.The classical approximation method (s=∞) is proved inapplicable,especially for alculation of the Curie temperature T_(c). As for theequation m(τ), there has been no equation m(τ) solely based onexperiment being able to describe all ferromagnetic materials (namelylaw of corresponding state) effectively in the past half century, whichcan be explained by a theory based on molecular field that m(τ) merelydepends on a dimensionless parameter.In the past, except for τ→0 and τ→1, no common analytical expression candescribe equation m(τ) in the molecular approximation field. However, anaccurate expression of equation m(τ) when 0<τ<1 derived by two or threesimple energy theorems is published recently, namelym(τ)=[1−sτ^(3/2)−(1−s)τ^(P)]^(1/3), where m is normalized spontaneousmagnetization,

${m \equiv \frac{M_{s}}{M_{0}}},$

M, is spontaneous magnetization, M_(o) is spontaneous magnetization atthe absolute zero temperature, M_(o)=M_(s)(T=0), τ is normalizedtemperature,

${\tau \equiv \frac{T}{T_{c}}},$

T_(c) is the Curie temperature, s and p are coefficients, p>3/2 and s>0.The equation follows Bloch's 3/2 energy law in low-temperatu region. Itcan be derived from critical state of the Heisenberg model that whenτ→0,

${m \approx {1 - {\frac{1}{3}s\; \tau^{3\text{/}2}}}},$

and m≈(1−τ)^(1/3) in the critical area (namely τ→1).Based on the above technical thoughts, the present invention provides anoninvasive temperature measuring method under a DC excitation magneticfield based on relationship between saturation magnetization offerromagnetic particles and temperature featuring high time definitionand high temperature definition. As in FIG. 1, the method comprisessteps of:(1) positioning ferromagnetic particles at a measured object;A small amount of ferromagnetic particles are placed inside the measuredobject or coated on the surface of the measured object by certain methodso as to not affect the appearance and normal functioning thereof.(2) applying a DC magnetic field to area of the ferromagnetic particlesenabling the ferromagnetic particles to reach saturation magnetizationstate;A constant DC magnetic field H_(dc)=b is applied to area of theferromagnetic particles enabling the ferromagnetic particles to reachsaturation magnetization state. Amplitude of the DC excitation magneticfield having the ferromagnetic particles reach saturation magnetizationstate is different for different materials.(3) obtaining steady temperature T₁ of the measured object at roomtemperature, and calculating initial spontaneous magnetization M₁, ofthe ferromagnetic particles according to the steady temperature T₁;Steady temperature T₁ of the measured object at room temperature isobtained by device such as a thermocouple or an optical fibertemperature sensor. Saturation magnetization-temperature curve of theferromagnetic particles is shown in FIG. 2. In the condition that theferromagnetic particles reach saturation state, spontaneousmagnetizations thereof are in one-to-one correspondence with thetemperature, and therefore the initial spontaneous magnetization M₁, ofthe ferromagnetic particles at temperature T₁ can be calculated.(4) detecting amplitude A of a magnetization variation signal of theferromagnetic particles after temperature of the measured object varies,and calculating temperature-after-variation T₂according to the amplitudeA of the magnetization variation signal;When particles are ferromagnetic, relationship between spontaneousmagnetization thereof and the temperature are determined, namelyequation m(τ)=[1−sτ^(3/2)−(1=s)τ^(P)]^(1/3), whereby temperature T ofthe measured object can be obtained, where m is normalized spontaneousmagnetization,

${m \equiv \frac{M_{s}}{M_{0}}},$

M_(s) is spontaneous magnetization, M_(o) is spontaneous magnetizationat the absolute zero temperature, M_(o)=M_(s)(T=0),τ is normalized temperature,

${\tau \equiv \frac{T}{T_{c}}},$

T_(c) is the Curie temperature, s and p are coefficients, p>3/2 and s>0.The equation follows Bloch's 3/2 energy law in low-temperature region.It can be derived from critical state of the Heisenberg model that whenτ→0,

${m \approx {1 - {\frac{1}{3}s\; \tau^{3\text{/}2}}}},$

and m≈(1−τ)^(1/3) in the critical area (namely τ→1).Therefore, temperature T₂ after change can be calculated according tospontaneous magnetization M₂ after the temperature variation. However,it is impossible to detect M₂ directly, instead, amplitude A of themagnetization variation signal and the corresponding duration Δt of thetemperature changing process are to be detected to obtain ^(M) ₂.Rapid temperature measurement is reflected in the time resolution.Temperature variation with changing duration on a nanosecond scale isimposed on the measured object and amplitude of the response signalthereof and the duration are detected by a test system.Two identical single-layer coils are used as sensors to detectmagnetization variation signals of the ferromagnetic particles in themeasured area. One inductance coil ^(α)is used as a detecting coil, themeasured object is contained therein so that the coil can detect allmagnetization variation signals of the measured object, and the otherinductance coil γ is placed at a symmetrical position in the DC magneticfield using as a reference coil, which receives noise in thecircumstance instead of induction signals of the measured object. Anequivalent model of an inductance coil at high frequencies is shown inFIG. 3. The inductance coil equals an inductance in series with aresistance in parallel with a capacitor and a transfer function thereofis

$\frac{1}{{s^{2}{LC}} + {sRC} + 1}.$

For an inductance coil with R=5Ω, L=800pμH and C=20pF, anamplitude-frequency response thereof is shown in FIG. 4. When resonantfrequency is about 1.2 MHz, temperature variation signal is inevitablydisturbed for rapid temperature measurement, especially for duration ofthe temperature changing process less than 1 us, namely a frequencythereof is higher than 1 MHz. Therefore, resonant frequency of theinductance coil should be improved to expand a normal operating range ofrapid temperature measurement. Resonant frequency of an inductance coilcan be improved by reducing turns thereof, however, distributioncapacitance and inductance thereof are reduced simultaneously whichreduces the induction signal response. Therefore, in the condition ofassuring an appropriate size of the output signal, using single-layercoils can meet the requirements of both resonant frequency and amplitudeof the induction signal. A single-layer coil has high resonantfrequency, but is vulnerable to environmental noise with small responseand low SNR (Signal Noise Ratio), which is unfavorable for signalextraction. A high-speed instrument amplifier is applied herein toamplify the two signals differentially so as to suppress common modeinterference and increase SNR. A high-speed data acquisition apparatusis applied herein correspondingly.Collecting magnetization variation signals of the ferromagnetic particlereagent in the measured area is carried out as follows. Output signalsof the system are constituted by circuit noise and spatial interferencewhen no heat source is applied, and when temperature starts to change, aheat source generates a heat change with a short duration of Δt in theferromagnetic particle reagent. Inductance coil ^(α)is used to collectmagnetization variation signals of the ferromagnetic particles, whichare processed by a conditioning circuit including a differentialamplification circuit along with signals detected by inductance coil γ.The processed signals are collected by a data acquisition card andstored in a computer for subsequent data processing, whereby obtaining amagnetization variation-time curve of the ferromagnetic particles and awaveform of a response signal. Therefore, output amplitude A of eachmagnetization variation signal processed by the conditioning circuit andits corresponding changing duration Δt are detected.Amplitude A of a collected signal is reverted to induce electromotiveforce ε, namely

${ɛ = \frac{A}{\beta}},$

where β is amplification factor of the conditioning circuit. Accordingto Faraday's law of electromagnetic induction,

${ɛ = {{- N}\frac{\Delta\Phi}{\Delta \; t}}},$

where ε, is induced electromotive force, N is turns of an inductancecoil, ΔΦis magnetic flux variation, Δt is the duration a change takes.Magnetic flux ariation ΔΦ can be calculated, and 66Φ=ΔB*S and ΔB=α*αM,where ΔB is magnetization variation, S is area, a is proportionalfactor, and ΔM is spontaneous magnetization variation, which can beobtained thereby. A correction value ΔM_(c)of the spontaneousmagnetization variation due to deviation caused by DC drift of the coilcan be obtained by performing median filtering of edge optimization onspontaneous magnetization variation ΔM, whereby obtaining a transientvalue M₂=M₁+M_(c) of spontaneous magnetization after changing with thetemperature.An expression

${M\left( t_{n} \right)} = {{M\left( {T = 0} \right)} \cdot \left( {1 - {s\left( \frac{T}{T_{c}} \right)}^{\frac{3}{2}} - {\left( {1 - s} \right)\left( \frac{T}{T_{c}} \right)^{\frac{5}{2}}}} \right)^{\frac{1}{3}}}$

is derived according to equation m(τ)=[1−sτ^(3/2) ⁻⁽1 s)τ^(p)]^(1/3), onwhich inverse calculation is performed to obtain a temperature T₂ , andtemperature variation ΔT=T₂=T₁, where t_(n) is time of a nth samplingpoint, M(t_(o)) is calculated by an initial temperature, s is aparameter of the thermal demagnetization curve of a ferromagneticmaterial, M(T=0 is spontaneous magnetization of the ferromagneticparticles at absolute zero temperature, T_(c) is Curie temperature ofthe ferromagnetic particles, and M(T=0) and T_(c) are determined for adefined material. Therefore, a relationship between M₂ and T₂ can bederived as follows:

$M_{2} = {{M\left( {T = 0} \right)} \cdot {\left\lbrack {1 - {s\left( \frac{T_{2}}{T_{c}} \right)}^{\frac{3}{2}} - {\left( {1 - s} \right)\left( \frac{T_{2}}{T_{c}} \right)^{\frac{5}{2}}}} \right\rbrack^{\frac{1}{3}}.}}$

A relationship between the temperature after change and the amplitude ofthe detected signal can be derived according to the above derivation:

${A = {\frac{a\; \beta \; {NS}}{\Delta \; t}*\left\{ {{{M\left( {T = 0} \right)} \cdot \left\lbrack {1 - {s\left( \frac{T_{2}}{T_{c}} \right)}^{\frac{3}{2}} - {\left( {1 - s} \right)\left( \frac{T_{2}}{T_{c}} \right)^{\frac{5}{2}}}} \right\rbrack^{\frac{1}{3}}} - M_{1}} \right\}}},$

where A is amplitude of the magnetization variation signal after thetemperature variation detected by the coils, T₂ istemperature-after-variation, ⁶⁰ is the proportional coefficient ofmagnetization variation ΔB to spontaneous magnetization variation ΔM, βis amplification factor of a test circuit, N is turns of an inductancecoil, S is inner area of the inductance coil, Δt is duration of thetemperature changing process, M(T=0) is spontaneous magnetization of theferromagnetic particles at absolute zero temperature, s is a parameterof the thermal demagnetization curve of a ferromagnetic material, T_(c)is Curie temperature of the ferromagnetic particles, M(T=0) and T_(c)are determined for a defined material, and M₁ is the initial spontaneousmagnetization of the ferromagnetic particles at temperature T_(1. ()5)calculating temperature variation ΔT=T₂−T₁ according to thetemperature-after-variation T₂ and the steady temperature T₁. Duration66 t of the rapid temperature changing process can be detected directlyby a test system. When multiple temperature variation occur, thetemperature variation equals a superposition thereof.

In experiment, for temperature variation on nanosecond scale or beloware rare in natural environment, thermal pulses generated by an opticalfiber laser are applied to the measured object in engineering to producefast temperature variation. A thermocouple is placed in the sametemperature environment as the measured object using as a temperaturereference apparatus simultaneously. Temperature changing states (namelytemperature changing environment) are provided by an optical fiber laseror other heat source. Duration is represented by t (on nanosecondscale), and power is represented by P. The optical fiber laser for theexperiment can generate a pulsed laser beam with a power of 0˜20W, apulse width of 200 ns, a rising time of 130 ns and a frequency of 23.3kHz. When output of a laser is focused by a lens, power density thereofis extremely large. Therefore, due to restriction of the Curietemperature of the ferromagnetic particles, unfocused output is usedwith a spot size of about 6 mm, which is able to heat the surface of themeasured object uniformly.

Response to a single laser impulse detected by a photovoltaic powerdiode is shown in FIG. 5 and ferromagnetic particles' response to asingle thermal impulse detected by a detecting coil is shown in FIG. 6.According to the two figures, rising time thereof approximately equals astandard rising time (130 ns) of a laser output by the laser, namely thetest system can clearly distinguish temperature variation in at least130 ns.Response to a consecutive thermal change by laser in 1 ms detected bythe test system is shown in FIG. 7. The total time is 2 ms. It is clearthat there are more than twenty laser impulse responses, amplitude ofeach of which can reflect power of a laser impulse, namely temperaturevariation caused by laser. Amplitude differences of the responseimpulses at the head and the tail are caused by power instability in theon-and-off process of the laser. Corresponding temperature variationdetected by a thermocouple is shown in FIG. 8. It can be inferred thatthe temperature variationd by about 0.03 ° C. and it is completelyimpossible to recognize temperature variation produced by each of theimpulses. Only a total temperature variation in 1 ms can be reflected.FIG. 9 shows temperature variation after analyzing response informationdetected by the test system via signal processing algorithms by magneticmeasurement, which is compared with that detected by a thermocouple inFIG. 10. It is clear that total temperature variation measured by thetwo methods are almost identical. However, temperature variation byevery laser impulse can be clearly recognized by magnetic measurement,namely magnetic measurement is far superior to thermocouple ontemperature definition and time definition. FIG. 11 shows difference oftemperature detected by magnetic measurement and by a thermocouple. Themaximum temperature error is 0.01° C.While preferred embodiments of the invention have been described above,the invention is not limited to disclosure in the embodiments and theaccompanying drawings. Any varies or modifications without departingfrom the spirit of the invention fall within the scope of the invention.

1. A noninvasive method for measuring rapid temperature variation of ameasured object under a DC excitation magnetic field, comprising stepsof: (1) positioning ferromagnetic particles at an area of said measuredobject; (2) applying said DC excitation magnetic field to said area,thereby enabling said ferromagnetic particles to reach saturationmagnetization state; (3) obtaining steady temperature T₁ of saidmeasured object at room temperature, and calculating initial spontaneousmagnetization M₁ of said ferromagnetic particles according to saidsteady temperature T₁; (4) detecting amplitude A of a magnetizationvariation signal of said ferromagnetic particles after temperature ofsaid measured object varies, and calculating temperature-after-variationT₂ according to said amplitude A of said magnetization variation signal;and (5) calculating temperature variation ΔT=T₂−T₁ according to saidtemperature-after-variation T₂ and said steady temperature T₁.
 2. Themethod of claim 1, wherein in said step (1), said ferromagneticparticles are placed inside said measured object or coated on thesurface of said measured object.
 3. The method of claim 1, wherein insaid step (2), said DC excitation magnetic field is applied to said areaby a Helmholtz coil.
 4. The method of claim 1, wherein in said step (3),said steady temperature T₁ of said measured object at room temperatureis obtained by a thermocouple or an optical fiber temperature sensor,and wherein said initial spontaneous magnetization M₁ of saidferromagnetic particles at temperature T₁ is calculated according to acurve between saturation magnetization and temperature of saidferromagnetic particles.
 5. The method of claim 1, wherein in said step(4), said calculating temperature-after-variation T₂ is performed by:calculating said temperature-after-variation T₂ according to arelationship between said temperature-after-variation T₂ and saidamplitude A of said magnetization variation signal:${A = {\frac{a\; \beta \; {NS}}{\Delta \; t}*\left\{ {{{M\left( {T = 0} \right)} \cdot \left\lbrack {1 - {s\left( \frac{T_{2}}{T_{c}} \right)}^{\frac{3}{2}} - {\left( {1 - s} \right)\left( \frac{T_{2}}{T_{c}} \right)^{\frac{5}{2}}}} \right\rbrack^{\frac{1}{3}}} - M_{1}} \right\}}},$wherein α is proportional coefficient of magnetization variation ΔB tospontaneous magnetization variation ΔM, β is amplification factor of atest circuit, N is turns of an inductance coil, S is inner area of theinductance coil, Δt is duration of temperature changing process, M(T=0)is spontaneous magnetization of the ferromagnetic particles at absolutezero temperature, s is a parameter of thermal demagnetization curve of aferromagnetic material, T_(c) is Curie temperature of the ferromagneticparticles, M(T=0) and T_(c) . are determined for a defined ferromagneticparticle material, and M₁ is the initial spontaneous magnetization ofthe ferromagnetic particles at temperature T₁.
 6. The method of claim 1,wherein in said step (4), said detecting amplitude A is performed by:detecting said magnetization variation signal of said measured objectusing a first single-layer coil, the measured object contained withinsaid first single-layer coil; detecting noise in circumstance using asecond single-layer coil, the second single-layer coil placed at asymmetrical position in the DC excitation magnetic field; wherein thefirst single-layer coil and the second single-layer coil are identical;and processing said magnetization variation signal along with said noisein circumstance by a conditioning circuit having a differentialamplification circuit, thereby detecting amplitude A of the processedmagnetization variation signal.